Approaches to Geological Observation

McMahon (1985), in his Davis Memorial Lecture, says
".... It has always seemed to me that uncertainty is the very essence of geotechnical engineering. Our materials are natural in origin, often irregular in form and highly variable in their properties. They are usually obscured from sight and can be investigated only to a small extent at great expense. The resulting uncertainty is often large and can have enormous economic consequences, ....."
He then goes on to develop the theme of geotechnical uncertainty, of which his first uncertainty is the risk of encountering an unknown geological condition.
Much has been written on acquiring geological knowledge of the site. Most publications concerned with site investigation recommend, albeit usually loosely, the practice of making regional and local desk studies, followed by specific investigations of the local geology. Some also counsel continuing to develop an understanding of the geology as project implementation proceeds. Hoek and Bray (1977), in their authoritative work on rock slope engineering, say, in the context of regional geological investigations,
"A frequent mistake in rock engineering is to start an investigation with a detailed examination of drill cores. While these cores provide essential information, it is necessary to see this information in the context of the overall geological environment.... and it is therefore useful to start an investigation by building up a picture of the regional geology." [Our bold for emphasis]

Professor Stapledon, the doyen of Australian engineering geology, has for at least the past three decades been advocating a carefully considered approach to site investigations. For example, Stapledon (1983) says that he believes that the geotechnical investigation should include some of the following geo-aspects which we have quoted:

Stapledon (1982), on dams (he notes that he was inspired by a similar suggestion by Peck,(1962), suggested that desirable attributes for those wishing to contribute to sub-surface engineering should include:

  1. Knowledge of precedents
  2. Knowledge of geology
  3. Knowledge of soil and rock mechanics

His accompanying recommendations for study of world and regional geological settings are reproduced here as Table 1.1. He introduces this table by saying that in order to determine a "semi-quantitative model (engineering-geological)", geological studies "should commence with consideration of the site location with respect to global tectonics, and include studies of the geology of a broad region surrounding the site. The main objectives and suggested activities for work on a regional scale are set out in [Table 1.1]. " He goes on to say, "The regional geological studies should be followed by studies on intermediate and detailed scales, a principal purpose of which is to ensure that the site geology "fits" into the regional geological picture, i.e. that the country adjacent to or containing the site is in situ, not displaced by major landslide or fault movements." [Our bold for emphasis]

We recommend the adoption of this first wise dictum, of starting with a broad regional understanding. However, this approach does not appear to be widely recommended. For example, the Australian Standard 1726 (1993), on Geotechnical Site Investigations, Item 4.7, says the process of evaluating the geotechnical character of a site "may include ... evaluation of the geology and hydrogeology of the site". It is silent on the construction of a geological model and on the understanding of regional geology and processes. The British Standard on Site Investigations, BS5930 (1981) has a somewhat similar silence. Even Peck's (1969) list quoted above, which starts with item (a) on 'Exploration', might be construed as starting without a desk study of the local and regional geology and his item (b) says only, 'In this assessment geology often plays a major rôle'.

We believe that in general the use of engineering geologists and geomorphologists and the application of intuitive and knowledge-based approaches which characterise good engineering geology do not occur at a level of involvement in projects that is commensurate with the worth of their contribution. In the authors' experience, it is not uncommon for engineering geologists to become involved after problems have developed rather than before.

failings relate to poor investigation technique and lack of geological understanding or assessment. Regrettably, the message from such studies often either falls on the ears of the converted or simply fails to reach the point of impact with the designers and those who award investigation contracts.